1. Field of the Invention
This application is a 371 of PCT/JP94/01360, Aug. 17, 1994.
The present invention relates to a process for hydrotreating a heavy oil containing, as impurities, metals such as vanadium and nickel and various compounds such as sulfur and nitrogen compounds, and to an apparatus employed therefor.
2. Description of the Prior Art
Processes employing a fixed bed (a), a suspended bed (b) and first a suspended bed and then a fixed bed (c) have been proposed for hydrotreating a heavy oil containing, as impurities, metals such as vanadium and nickel and various compounds such as sulfur and nitrogen compounds.
The above processes have the following drawbacks.
(a) Drawbacks of the process in which a heavy oil is hydrotreated with a fixed bed
The process having predominantly been employed for hydrotreating a heavy oil is one using a fixed bed. For example, this process comprises hydrotreating in a fixed-bed reactor having a first reaction chamber packed with a hydrodemetallization catalyst into which a heavy oil is fed to thereby hydrotreat the same and a second reaction chamber packed with a hydrodesulfurization catalyst in which the thus hydrotreated heavy oil is further hydrotreated.
However, when the removal of metals and sulfur and nitrogen compounds from a heavy oil is conducted to a high degree in a fixed-bed reactor, it has occurred that metals resulting from demetallization are converted to sulfides and deposit on the catalyst at the inlet part of the reactor to thereby deactivate the catalyst. Also, it has occurred that the outlet part of the reactor comes to have a high temperature due to the heat of reaction to thereby cause asphaltene at that part to suffer from thermal decomposition so as to produce coke which forms a solidified carbon compound known as a dry sludge to deposit on the catalyst, so that the catalyst is deactivated. Further, deposition of the dry sludge has occurred in pipes arranged downstream of the reactor.
Therefore, the process in which a heavy oil is hydrotreated with a fixed bed has had a drawback in that it is difficult to conduct the hydrotreating operation for a prolonged period of time.
(b) Drawbacks of the process in which a heavy oil is hydrotreated with a suspended bed
Known processes in which a heavy oil is hydrotreated with a suspended bed include the H-oil process.
When the hydrotreating of a heavy oil is conducted only with a suspended bed, although the reaction temperature can be kept uniform, there has been a drawback in that the efficiency of utilization of the catalyst is poor, so that the reaction temperature must be increased for reducing the sulfur and nitrogen content of the product oil to a low level with the result that thermal decomposition, rather than nuclear hydrogenation, is advanced to thereby degrade the product oil.
(c) Drawbacks of the process in which a heavy oil is hydrotreated first with a suspended bed and then with a fixed bed
This process comprises the steps of first hydrotreating a heavy oil with a suspended bed and then hydrotreating the resultant heavy oil with a fixed bed. This process is aimed at preventing the deactivation of the catalyst caused by deposition of metals on the catalyst so as to prolong the hydrotreating operation period.
This process has drawbacks similar to those of the process in which a heavy oil is hydrotreated with a fixed bed. For example, in this process, coke unfavorably forms a dry sludge, which deposits on the catalyst to thereby deactivate the catalyst. Therefore, in this process, it is difficult to prolong the hydrotreating operation period.
In any of the above conventional processes for hydrotreating a heavy oil, it is requisite to discontinue the hydrotreating operation every about 10 months and to replace the employed catalyst with fresh one. This replacement takes a period as long as 10 to 30 days when the apparatus is for commercial purposes.
The inventors have noted that impurities contained in a heavy oil such as compounds containing vanadium, nickel and other metals, sulfur and nitrogen compounds have different reactivities with hydrogen during hydrotreating depending upon the impurities contained in different heavy oil fractions, such as resin and asphaltene, and found that, when impurities contained in the asphaltene or the like and having low reactivities with hydrogen are forcibly removed together with impurities contained in the resin or the like and having high reactivities with hydrogen to a high degree during the hydrotreating in a fixed-bed reactor, the fractions containing impurities having low reactivities with hydrogen are converted to coke, which deposits on the catalyst to thereby deactivate the catalyst with the result that the long-term hydrotreating operation becomes difficult. The present invention has been completed on the basis of this finding.
The objective of the present invention is to provide a novel process for hydrotreating a heavy oil, which permits prolongation of the hydrotreating operation period, and to provide a novel apparatus suitable therefor.